The present invention relates to a method for testing aspherical surfaces. The method is based upon the principle that an aspherical surface is composed of various spherical surfaces of different radii of curvature.
There are many interferometric methods available nowadays for measuring the objects having an aspherical surface. The interferometric methods can be classified into null tests and non-null tests. Neither of these methods, however, can effectively measure the shape of an aspherical object. The null tests have the following disadvantages:
(1) To measure different objects having an aspherical surface, specially designed optical devices are needed. Therefore, the cost is extremely high and it is difficult to build such devices; and
(2) The compensating effects of the null optics cannot be independently measured.
With respect to the non-null tests, the disadvantages are as follows:
(1) The density of interferograms at the edge of the measured object is generally larger than the resolution which can be offered by the image capturing device; and
(2) Generally an optical system is designed for one spherical surface. If many spherical surfaces are used simultaneously, aberrations occur in the optical system itself.
Subaperture optical testing techniques have been developed to solve the above problems. The techniques determine the deviation of the tested aspherical surface from a referenced spherical surface at each movement of the aspherical surface and calculates segmental profiles of the aspherical surface according to the deviation at each movement. The whole profile of the aspherical surface is calculated by combing the segmental profiles. The calculation is complex since it should determine each segmental profile of the aspherical surface.
The reference entitled "Profile Measurement of an Aspherical Cylindrical surface from Retroreflection" by Ding-Tin Lin and Der-Shen Wan, published on APPLIED OPTICS, Vol. 30, No. 22, Aug, 1, 1991 discloses a way to analyze an aspherical surface by measuring the fringe density of the interferogram reflected from the aspherical surface with an interferometer, and to scan a region in the interferogram having a smaller fringe density by changing the reflection angle. However, this method can only be used in analyzing cylindrical surfaces, but not for measuring an arbitrary objects with aspherical surface.